Recent concerted efforts in the field of liquid crystal materials have yielded a new class of reflective, cholesteric texture materials and devices. These liquid crystal materials have a periodic modulated optical structure that reflects light. These materials, known as polymer stabilized cholesteric texture (PSCT) and polymer free cholesteric texture (PFCT) are fully described in, for example, U.S. Pat. No. 5,251,048 and patent application Ser. Nos. 07/694,840 and 07/969,093, the disclosures of which are incorporated herein by reference.
Reflective cholesteric texture liquid crystal displays (both PSCT and PFCT) have two stable states at a zero applied field. One such state is the planar texture state which reflects light at a preselected wavelength determined by the pitch of the cholesteric liquid crystal material itself. The other state is the focal conic texture state which is substantially optically transparent. By stable, it is meant that once set to one state or the other, the material will remain in that state, without the further application of a field, as is the case in conventional displays. Conversely, in other types of conventional displays, each liquid crystal picture element must be addressed many times each second in order to maintain the information stored thereon. Accordingly, PSCT and PFCT materials are highly desirable for low energy consumption applications, since once set they remain so set.
Applications for PSCT and PFCT materials have focused on reflective displays. Typically such reflective displays have had the back plate thereof painted black to absorb any non-reflected light. As a result, the displays show the contrast of green, yellow, or such other color determined by the pitch of the cholesteric texture material, on black. In fact, black has heretofore been preferred since it can provide for high contrast ratios.
Conventional methods for addressing or driving such displays can be understood from a perusal of FIG. 1. FIG. 1 illustrates a graph showing the state of the liquid crystal material after the application of various driving voltages thereto. The liquid crystal material begins in a first state, either the reflecting state or the non-reflecting state, and is driven with an AC voltage, having an rms amplitude above V.sub.4 in FIG. 1. When the voltage is removed quickly, the liquid crystal material switches to the reflecting state and will remain reflecting. If driven with an AC voltage between V.sub.2 and V.sub.3 the material will switch into the nonreflecting state and remains so until the application of a second driving voltage. If no voltage is applied, or the voltage is well below V.sub.1, then the material will not change state, regardless of the initial state. In the context of a conventional segmented display, the background is always at a zero voltage. The segments are addressed with either low or high voltages to drive the segments to either black (non-reflecting) or color (reflecting).
The conventional method of driving PSCT and PFCT displays is described in an article entitled "Front-Lit Flat Panel Display from Polymer Stabilized Cholesteric Textures", by Doane, et al. and published in Conference Record, page 73, Japan Display '92, Society of Information Displays, October 1992 (the "Doane Article"). The Doane Article teaches addressing a row in a display by applying an AC waveform with an rms amplitude V.sub.rs between V.sub.2 and V.sub.3. A column voltage of zero is applied to the columns of all the pixels in the rows which are to be in the non-reflecting state. An AC voltage with rms amplitude greater than or equal to V.sub.4 -V.sub.rs, but less than V.sub.1 is applied to the columns of all pixels which are to be in the reflecting state.
The column voltages are out of phase with respect to the row voltages so that the effective voltage across the selected pixels is greater than or equal to V.sub.4. The amplitude of the column voltage is always less than V.sub.1, thus as the addressing of the display progresses from row-to-row, the column voltage does not alter the state of the pixels in rows which have already been addressed. Specifically, for a given single pixel, at time when no voltage is applied to the row address line of the display for the pixel, and a column voltage of V.sub.c (either + or -). The result is no change in the pixel since the pixel's row was not selected. During time when no voltage is applied to either the row or column lines for the pixel, and again the pixel is unchanged.
An improved driving scheme is described in U.S. patent application Ser. No. 08/288,831, filed Aug. 11, 1994, and entitled DRIVING METHOD FOR POLYMER STABILIZED AND POLYMER FREE LIQUID CRYSTAL DISPLAYS, by Catchpole, et al. The improved driving scheme allows for faster addressing of cholesteric liquid crystal displays, while reducing optical artifacts, and residual effects.
Referring now to FIG. 2, there is illustrated therein a partial cross-sectional side view of a PSCT or PFCT display device. The display 1 includes a first display substrate 2 fabricated of an insulating material such as glass, plastic or some other polymeric material. The substrate 2 has first and second major surfaces 3 and 4. On the first major surface 3 of substrate 2 is disposed a layer of an electrically conductive material 5. The electrically conductive layer 5 should be a transparent material. Accordingly, the electrode layer 5 may be a thin layer of metal such a silver, copper, titanium, molybdenum, and combinations thereof, so long as the metals are very thin, and non-reflective. Alternatively, the layer 5 maybe a thin layer of a transparent conductive material such as indium tin oxide. The layer may be fabricated as a plurality of elongated strips on the surface of the substrate 2.
Disposed opposite the first substrate 2 is a second substrate 6 fabricated of a high quality, transparent material such a glass or plastic. Disposed on one surface is an electrode 7, fabricated of a transparent conductive material, such as those described hereinabove with respect to layer 5.
The substrates 2 and 6 are arranged in opposed, facing relationship so that said layers of conductive material are parallel and facing one another. Disposed between said layers of conductive material is a layer of PSCT or PFCT liquid crystal material 8. The liquid crystal material has a periodic modulated optical structure that reflects light. The liquid crystal material comprises a nematic liquid crystal having positive dielectric anisotropy and chiral dopants. The material may further include a polymer gel or dye material. Thus, an electrical field may be applied to a layer of PSCT or PFCT liquid crystal material disposed therebetween. Once such a field is removed, the material is set to one of two said stable states, where it will remain until a new field is applied
Notwithstanding the advantages provided by the bistable nature of PSCT and PFCT displays, they have not heretofore been useful in the field of segmented displays. FIGS. 3 and 4 are respectively, representations of the segmented and common electrode substrates of a segmented display device of the prior art. In FIGS. 3 and 4, areas which appear dark, such as segments 12, 14, and 16, lead lines 18, 20, and 22, and contacts 24, 26, and 28 of FIG. 3, and electrodes 30, 32, and 34, are all regions coated with a thin layer of a transparent conductive material such as indium tin oxide (ITO). Essentially, electrodes in the shape of the desired image are patterned onto a substrate.
Unfortunately, due to the nature of PSCT and PFCT materials, the conventional design will not work since the lead lines 18, 20, 22 and the areas without ITO will appear different than the other ITO coated areas. Attempts have been made to overcome this by masking the entire display (except the active segments) with various materials, such as black epoxy. This has yielded unacceptable results for some applications since it causes parallax at larger viewing angles. Further, once a mask color is chosen to match one of the states of the material, for example black, the display mode cannot be changed. For example a display which is assembled for yellow on black cannot be used as a black on yellow display.
Thus, there exists a need for a segmented, color PSCT and PFCT liquid crystal displays. In providing such a display, it should not have a great deal of parallax, yet should be able to provide for multiple color modes of operation. The device should also have a simplified driving scheme to easily and efficiently address and re-address the segmented display.